The function of ultrasonic power converters is based on the reversal of the piezoelectric effect. By application of a voltage to the opposite sides (hereinafter referred to as upper and lower sides) of a plate of piezoelectric material, the plate is caused to ultrasonically oscillate. For this purpose, plates of piezoelectric material are provided with a complete electrically conductive coating, preferably a silver coating, on their upper and lower sides, with one wire each, connected to a voltage source, being soldered onto the same. The coating of the upper and lower sides of the plate is necessary, as the electric voltage would otherwise only be more or less punctually transferred to the piezoelectric material at the soldered joint. Contrary to this, with the coating the voltage is transferred onto the entire upper and/or lower side of the plate.
One known form of application of ultrasonic power converters is their use in quartz clocks.
In addition, ultrasonic power converters form the essential part of medical sterilisation devices in which liquid is present on one side of the plate of piezoelectric material. With the application of a voltage to opposite sides of the plate the liquid is caused to oscillate. The quickness of the oscillating liquid produces cavitation zones which produce very high temperatures upon imploding. Germs or viruses in liquids are thereby killed.
The applicants themselves use ultrasonic power converters according to DE41 01 303 Al of Poschl et. al. for the high-pressure atomization of liquid fuel which was published on Jul. 30, 1982 after the claimed priority data. In this connection the plate of piezoelectric material is fixed to an aperture in a wall. Fuel impinges on the opposite sides of the plate, i.e. on both sides, in order for the fuel to be foamed by means of the cavitation zones produced by the plate oscillating at ultrasonic frequency. Decisive for the function of the plate of piezoelectric material is that its upper and lower sides be in complete liquid-tight isolation from one another and that, furthermore, the upper and lower sides be electrically isolated from one another. Otherwise, liquid connecting the upper with the lower side would act as an electric bridge for the voltage applied between the upper and lower sides, which would cause a short circuit.
To avoid this short circuit, according to the internal state of the art of the applicant, for a holder it has already been proposed to provide the plate of piezoelectric material with a seal of electrically insulating material at its periphery, whereby the upper and lower sides of the plate are sealed liquid-tight and electrically isolated from one another. In addition, a holding means is used to fix the plate at the aperture in the wall. The wall in this case divides a container into which the liquid fuel is conducted into two chambers. The chambers communicate with one another via a long fuel line.
The problem of liquid-tight sealing exists not only when liquid impinges on both sides of plates of piezoelectric material, but also when liquid impinges only on one side.
In the holder according to the aforementioned older proposal of the applicant it has been shown that problems with the liquid-tight sealing could not be ruled out and short circuits frequently occurred. Moreover, the soldered joints at which the electric lines were connected to the associated upper and lower sides of the plate very frequently broke, since the plate oscillates at ultrasonic frequency and is thus constantly in motion. Another disadvantage is that a silver coating of the plate tends to oxidate.